1. Field of the Invention
This invention relates to a medical stereo observation system suitable for the stereo observation of a medical image used in a surgical operation, and particularly in neurosurgery, otolaryngology, orthopedic and plastic surgery, obstetrics and gynecology, or ophthalmology.
2. Description of Related Art
In recent years, the technique of using the medical stereo observation system, such as a stereoendoscope or a surgical (stereo) microscope, has been popularized in the field of medical treatment (notably surgery). In general, the medical stereo observation system is of a binocular type, and includes a stereo imaging unit incorporating imaging optical systems and image sensors to form left and right images with parallax of an observation object, a stereo image signal processing unit in which a stereo image is produced by the image signals of the left and right images derived from the stereo imaging unit, and a stereo display unit displaying the stereo image of the stereo image signal processing unit. In the medical stereo observation system, the images of the object are formed on the imaging surfaces of the image sensors by the imaging optical systems of the stereo imaging unit. In order to obtain the left and right images with parallax, various techniques are used in the imaging optical systems.
The left and right images obtained by the image sensors are transmitted as the image signals from the stereo imaging unit to the stereo image signal processing unit. The stereo image signal processing unit performs signal processing necessary for the stereo display unit provided behind the signal processing unit. The stereo display unit forms the left and right images on display elements on the basis of the stereo image sent from the stereo image signal processing unit. In order to separately transmit the left and right images to a viewer's eyes, various techniques are also used in the stereo display unit.
As a typical example of the technique of a stereo display unit, there is a virtual-image stereo observation type in which light is projected directly on the left and right pupils (eyes) of the viewer corresponding to the left and right images by optical systems located very closed to the viewer's face so that image information of a large image plane is equivalently observed stereoscopically as a virtual image. A conventional technique of a system combining the stereoendoscope with the virtual-image stereo observation type stereo display unit is set forth, for example, in Japanese Patent Kokai No. Hei 8-313828. This stereo observation system establishes the relationship between the imaging field angle of the stereoendoscope and the image observation field angle of the virtual-image stereo observation type display unit in order to observe the image of the stereoendoscope in a natural, virtual reality.
Such a conventional stereo observation system, as mentioned above, includes an imaging section producing images with parallax and an image display section displaying the images with parallax produced in the imaging section to make the viewer to fuse stereoscopically the images. The imaging section, for example, as disclosed in Japanese Patent Kokai No. Hei 8-201940, has two imaging optical systems for left and right eyes and adjusts the focal positions of the imaging optical systems for left and right eyes to a position required for the observation on an observation object to produce the images with parallax. In this case, the focal positions of the imaging optical systems for left and right eyes coincide with an intersection of their optical axes.
Various display devices of the stereo images used as the image display sections are known. For example, a display device capable of making the stereo image observation without mounting spectacles provided with observation optical systems and shutter functions on the face is disclosed in Japanese Patent Kokai No. 2003-233031. In an image display device disclosed here, images for left and right eyes are projected on a display panel surface by projection optical systems for left and right eyes so that the left and right images are collected at the positions of left and right eyes of the viewer and thereby collected left and right images with parallax are observed and fused to make the stereo observation. In this image display device, the optical axes of the projection optical systems for left and right eyes cross on the display panel surface and are reflected. Light of the projection optical systems for left and right eyes, reflected by the display panel surface is collected at the positions of the left and right eyes of the viewer. In this case, the focal positions adjusted to the position required for the observation on the observation object, produced by the imaging section are reproduced on the display panel surface.
On the other hand, in conventional neurosurgery, a surgical microscope is used in order to carry out a magnification observation on a fine part to be operated. In organs including fine tissues, such as brains, it is difficult to recognize their structural tissues with the naked eye and thus the treatment of organs is made under the microscope. Moreover, the operation of neurosurgery is such as not only to make observations on very important and delicate tissues like blood vessels and nerves in a very narrow region, but also to make the treatment that blood vessels or nerves are connected or blood vessels and nerves are avoided to remove a tumor. For this, it becomes important to make the stereo observation. In such medical circumstances, the observation object, such as a part to be operated, of brains, often has a hole-like and deep shape. Here, consider the case where the part to be operated is observed by the stereo observation system mentioned above. When the focal positions are imaged together with the bottom of the hole of the part to be operated, an image located on the front side of the bottom of the hole of the part to be operated, is reproduced in front of the display panel surface.
Generally, in the parallax of the images for left and right eyes, there is a limit to the image fusion with the left and right eyes of the viewer. The amount of parallax at a position separated by the depth from a plane of a desired depth position (the focal plane) on the observation object to which the focal positions are adjusted becomes large as it separates from the focal plane. Consequently, in a stereoscopic space reproduced on the display panel, a space limit by the amount of parallax that is an image fusion limit occurs.
When the hole-like part to be operated is observed in the conventional stereo observation system, the part to be operated is reproduced only in front of the display panel, and thus an image reproducing space situated on the far side (the inmost side) of the display panel cannot be effectively used. In addition, when a deep hole such as that reaching a deep portion of the brain is observed, an image close to the entrance of the hole is reproduced at a position where the image fusion limit is exceeded, and hence this is responsible for the fatigue of the eyes when the viewer makes the observation.
In the stereo observation system, when electronic zoom is applied during the observation of the observation object with a depth, the amount of parallax on the display panel surface of the observation object at a position separating from the focal positions of the imaging optical systems for left and right eyes, imaged by the imaging section is increased in accordance with the electronic zoom, and the image fusion of the viewer becomes difficult. In order to prevent this, as disclosed in Japanese Patent Kokai No. Hei 9-37302, a stereo imaging apparatus is proposed in which a distance between the observation object and the focal position is calculated and the position of a display image is shifted in accordance with the distance so that the image fusion is easily attained.